Background Art
Current packaging techniques for semiconductor chips include various types of lead frame packaging. In such lead frame types of packaging a sheet of conductive material such as copper is provided which is punched or etched or otherwise formed to a configuration wherein a plurality of fingers are provided which are attachable to pads formed on a semiconductor chip. The limitation of how closely the fingers can be spaced for both etching and punching techniques is a function of the thickness of the lead frame material. There are practical limitations to how thin this stock can be, established by the fragility and planarity requirements needed to attach the finished module to the panel (card or board) in subsequent assembly processes. This dictates that the lead frame thickness be generally in the range of 4 to 10 mils, depending on the metal selection of this lead frame. The chip pads provide for the input/output, ground and voltage leads to the circuitry on the chip. In one form of lead frame packaging the fingers are directly attached to the contact pads on the chip. However, as the size of semiconductor chips decreases while the same amount of circuitry is maintained or even increased, thus maintaining or even increasing the number of I/O pads, the physical size of the fingers which would be required to provide the necessary signals and other physical characteristics including the required spacing between the fingers is such that it is not possible to directly attach to the chip to all the fingers that are required for the various input/output, voltage and ground connections on the chip.
One technique for overcoming the size limitations of the fingers is to form the lead frame in such a way that the fingers terminate a substantial distance from each edge of the chip thus providing a large enough geometry of the inner periphery of the lead frame, which surrounds the chip, to allow for the formation of the required number of fingers to connect with the input/output, voltage and ground pads. The input/output pads of the chip are then joined to the fingers by means of lead wires which are electrically interconnected between the pads on the chip and the fingers such as by soldering or other electrical bonding means. This type of connection is reasonably adequate over relatively short distances (i.e., up to about 100 mils or so). However, over longer distances (e.g., distances approaching 200 mils) this type of connection has several disadvantages.
One particular disadvantage is encountered during subsequent encapsulation processes wherein the chip, the lead frame, and connector wires are molded into a plastic encapsulant to form a final structure. During this molding or encapsulating operation, the plastic, which is viscus, is caused to flow around the chip and the lead frame and connecting wires. One of the phenomena that takes place during this molding or encapsulating is what is known as "wire sweep" or "wire wash". This is characterized by a force generated by the relatively viscous encapsulant flowing against the wires, which force can cause wires to break their connections with either the chip or the fingers or in some cases can cause the wires to either short against each other or come into such close proximity so as to cause unwanted capacitance and other problems.
Another drawback to the connection of lead wires to the fingers and the chip is the increased inductance caused by the length of the wires necessary to form the connection.
In addition to these drawbacks, there is an additional limitation to conventional lead frame bonding. This drawback is characterized by the requirement that each finger be aligned with the specific connection pad on the chip to which it is to be connected. Expressed another way, this limitation dictates that the pads on the chip must be connected to the fingers on the lead frame in the same order that they are positioned on the chip; i.e., there can be no cross-over connections of the fingers to pads which are not directly aligned with them.
There have been several prior proposals for improving the connection of a chip to the fingers of a lead frame by means of connecting wires. Two such proposals are shown in Japanese Kokai 62-94967 to NEC Corporation and Japanese Kokai 61-237459 to Sumatomo Electric Ind. Ltd. Both of these references show a chip connected to the fingers of a lead frame in which insulating films are interposed between the chip and the fingers and the wires are bonded from the pads on the chip to the conducting pads on the insulating films and then in a second step from the conducting pads on the insulating film to the fingers. Japanese Kokai 61-82439 to Toshiba Corporation shows a similar structure but in this structure rather than the two wires being electrically bonded to an intermediate section a single wire is looped and connected physically by an adhesive to an intermediate section and then carried on to the finger.
European Patent Application 0078606 to Texas Instruments shows a technique of bonding chip pads to lead frame fingers wherein a single wire is connected both to the chip pads and the finger and is supported intermediate its ends by an insulating support.
All four of these references do address the problems of "wire sweep" or "wire wash" in that the unsupported lengths of the wires are reduced as opposed to in a single loop running from the pads on the chip to the fingers. However, none of these references address the problem of the increased inductance caused by the long wire lengths. Moreover, none of these references teach or allow for cross-over, and all require the connection of a specific finger with the aligned pad on the chip.